The present invention relates in general to compandors and, more particularly, to a compandor with a DC-coupled compressor.
A compandor is a combination compressor circuit and expandor circuit. The compressor circuit compresses the dynamic range of an analog input signal for transmission across a narrow-band transmission link, while the expandor circuit expands the compressed signal back to its original state at the receiver side. Typical applications include cordless telephones and intercom systems. The compressor narrows the dynamic range of the analog input signal by amplifying frequency components below a predetermined amplitude threshold while attenuating frequency components above the amplitude threshold. For example, if the amplitude threshold is established at -20 dBV and the compressor narrows the dynamic range by a 2-to-1 dB ratio, signal components in the -80 dBV range are amplified to -50 dBV while the higher level 0 dBV components are attenuated to -10 dBV. As a result, the amplitude of the analog input signal after compression ranges from -10 dBV to -50 dBV over the entire frequency range. Following transmission over the RF link, the compressed signal is expanded to its original range of 0 dBV to - 80 dBV.
One advantage of the compandor is that narrowing the dynamic range allows transmission over the RF link without clipping the frequency components with a higher amplitude. Transmission over a cordless telephone for example is typically limited to 400 millivolts peak-to-peak, while active voice signals usually range from 0 dBV to -30 dBV. A 0 dB signal would be clipped over the RF link thereby losing transmission data. Another advantage is that amplifying the low level frequency components of the input signal in the -40 to -60 dBV range raises the input signal above the noise level of the transmission link and improves the signal to noise ratio (SNR). The noise floor of a typical RF link is -50 dBv. Hence, the lowest level input signal of -80 dBV is amplified to at least the anticipated level of the transmission noise.
Compandor circuits in the prior art manufactured as a monolithic integrated circuit (IC) require packages with 12 or more pins to accommodate the necessary external components on a printed circuit board, i.e., large electrolytic capacitors. In the highly competitive market of cordless telephones, reducing the external pin count and package size can lead to reduced manufacturing costs, reduced space allocation and increased reliability.
Hence, a need exists for a compandor circuit requiring fewer external components and therefore manufacturable in a cheaper monolithic integrated circuit package with fewer external pins.